Labels bearing information in any of a number of different encoded formats are commonly affixed to products, packaging, or other items and are used in many applications. It is common, for example, to encode retail product identification information in a bar code format on a product package or a label affixed to a product package. Bar code symbols are also used on a broad range of retail packages for check-out-and inventory purposes. A bar code reading device, located at the check-out station of a retail establishment, may be used by a clerk to read product identification data into an associated point-of-sale computer system.
A number of existing bar codes consist of a series of parallel light and dark rectangular areas of varying widths. The light areas are often referred to as "spaces" and the dark areas as "bars". The bars and spaces are typically arranged and selected so as to define different characters of a particular bar code.
A bar code label may be read by a device (e.g., a scanner) which detects reflected and/or refracted light from the bars and spaces comprising the characters. Such a device typically includes a light source for illuminating the bar code label to be read. One common method of illuminating the bar code label is by use of a scanning laser beam, in which case a spot of light is swept across the bar code label and the intensity of returned light is detected by an optical detector. The optical detector generates an electrical signal having an amplitude determined by the intensity of the detected light. Another method for illuminating the bar code label is by use of a uniform light source with an array of optical detectors connected to an analog shift register (commonly called a charge-coupled device or CCD). In such a technique, as with a scanning laser, an electrical signal is generated having an amplitude determined by the intensity of the collected light. In either the laser or CCD technique, the amplitude of the electrical input signal has one population of levels for dark bars and a second population of levels for light spaces. As a label is scanned, positive-going transitions and negative-going transitions in the electrical signal occur, signifying transitions between bars-and spaces. The electrical signal may be converted to a binary scan signal which is analyzed to determine the arrangement of bars and spaces of the scanned label. The bar and space information is provided to a decoding unit to determine whether the bar code is recognized and, if so, to decode the information contained in the bar code.
An essential element of virtually every existing bar code reader system is a means for illuminating the bar code. Laser based scanners, for example, typically use a flying laser spot to illuminate the bar code and detect the returned light. Wand devices generally use a light emitting diode (LED) to illuminate a small portion of the bar code and a detector to image the bar code while the operator physically sweeps the illuminated beam across the bar code. Video based readers, such as readers utilizing charge-coupled devices to receive input, typically use a plurality of LED's to illuminate the entire bar code with uniform intensity, and then image the entire bar code with a CCD sensor.
Each of the above systems relies on self-generated illumination for the purpose of distinguishing the bars and spaces of the bar code or for increasing the amount of collected light to enable reasonable performance. The requirement of self-generated illumination may, however, lead to additional complexity, a larger or bulkier device, and/or higher power consumption. For example, a scanning device using a flying laser spot may require a patterned mirror structure, bifocal lens, laser light source, and motor to sweep the laser spot across a target. Each of these components adds complexity and bulk to the overall system. Furthermore, the motor used for sweeping the laser spot tends to be one of the least reliable components of a scanner, followed by the laser illumination source. The use of a motor and laser illumination source tends to make a system less reliable and more costly. In short, the requirement of a self-generated illumination generally makes an image reading system more complex, costly and unreliable. These drawbacks may be significant in certain applications, particularly in low-cost, handheld reading applications.
In addition, CCD readers often have significant depth of field limitations. For example, CCD readers using red LED's for illumination typically have a depth of field of less than one inch. This limits the types of applications in which CCD readers may be used, and limits the versatility of such CCD readers in general. Moreover, due to low sweep speeds, use of CCD readers has also generally been limited to handheld devices.
It would therefore be advantageous to provide a bar code reader capable of operation without self-generated illumination. It would further be advantageous to provide such a bar code reader having an increased depth of field, that may be utilized in a broad base of applications including handheld devices as well as other applications. It would further be advantageous to provide a bar code reader having a reduced number of moving parts and exhibiting increased reliability.